1. Field of the Invention
The present invention relates to a liquid crystal display device and more particularly to a liquid crystal display device which does not cause air bubbles or liquid crystal leakage and thus is improved in reliability in a configuration of filling liquid crystal between a TFT substrate and a counter substrate by a one drop fill method.
2. Description of the Related Art
In liquid crystal display devices, liquid crystal is filled between a TFT substrate on which pixels each having a pixel electrode and a thin film transistor (TFT) are formed in a matrix and a counter substrate on which color filters and the like are formed, and an image is formed by controlling the molecules of the liquid crystal by an electric field. The gap between the TFT substrate and the counter substrate is as extremely small as several microns. In the conventional filling method of liquid crystal, the space between the TFT substrate and the counter substrate is sealed to create a vacuum within the space, and liquid crystal is injected by means of atmospheric pressure.
However, when the gap between the TFT substrate and the counter substrate is small, and the display area of the liquid crystal display device is large, the injection requires a great deal of time, which lengthens manufacturing throughput and thus increases the manufacturing cost. For addressing the problem, for example, a technique of sealing liquid crystal has been developed in which a required amount of liquid crystal is dropped onto the counter substrate, and thereafter the counter substrate and the TFT substrate are overlapped and sealed.
Such a one drop fill method has been conventionally employed for relatively large liquid crystal display devices and has started to be used for small liquid crystal display devices. In small liquid crystal display devices, a number of liquid crystal cells are formed on a mother substrate, and liquid crystal has to be sealed in each of the liquid crystal cells. However, a number of man-hours are required for sealing liquid crystal into the individual liquid crystal cells. According to the one drop fill method, liquid crystal can be injected into a number of liquid crystal cells at one time in the mother substrate.
In the specification, while the “liquid crystal cell” refers to one in a state where the TFT substrate and the counter substrate are sealed with a sealing material, and liquid crystal is sealed therebetween, the “liquid crystal display device” refers to one on which a drive IC for driving liquid crystal is mounted on the liquid crystal cell. However, they are sometimes used with no distinction.
In liquid crystal display devices, it is important to control the gap between the TFT substrate and the counter substrate. Conventionally, the gap is controlled by columnar spacers formed on the counter substrate in a display region and controlled by glass fibers in a seal portion.
On the other hand, JP-A-2001-174827 discloses a configuration of a liquid crystal display device in which columnar spacers are used in a display region, and columnar spacers are used also in a seal portion. JP-A-2001-174827 describes a configuration in which columnar spacers are formed on BM both in the display region and in the seal portion, so that the gap between the substrates in the display region is the same as that of the seal portion.
On the other hand, the adhesion between the sealing material, and the TFT substrate and the counter substrate in the seal portion is important for reliability. JP-A-2007-212567 describes a configuration in which, for preventing the intrusion of liquid crystal between the sealing material and the lower surface of the TFT substrate or the counter substrate in the seal portion, a weir serving as a stopper against liquid crystal is formed.
FIG. 14 is a plan view showing a state of a mother substrate 1000 for manufacturing small liquid crystal display devices. In FIG. 14, the mother substrate 1000 is formed by overlapping a mother TFT substrate and a mother counter substrate. In the mother substrate 1000, 7×5=35 pieces of liquid crystal cells 1 are fabricated. In FIG. 14, scribe lines 2 for separating the mother substrate into the individual liquid crystal cells 1 are marked, and a sealing material 20 is formed for each of the liquid crystal cells 1. Liquid crystal is dropped in a region inside the sealing material 20 and sealed with the sealing material 20.
In small liquid crystal display devices, a glass substrate is required to be thin. A glass substrate serving as a mother TFT substrate or a mother counter substrate is standardized and is as thick as about 0.5 mm. Therefore, after forming the mother substrate 1000, the mother substrate 1000 is reduced in thickness by polishing the outer surface thereof. In this case, for preventing a polishing solution from entering the inside of the mother substrate 1000, a mother substrate sealing material 2000 is formed at the peripheries of the mother TFT substrate and the mother counter substrate. Thereafter, the individual liquid crystal cells 1 are separated from the mother substrate 1000 along the scribe lines 2.
In the liquid crystal one drop fill method, the amount of liquid crystal to be dropped is very important. When liquid crystal is dropped onto the mother counter substrate, an accurately controlled amount of liquid crystal is dropped in a region surrounded by the sealing material 20 formed in the individual liquid crystal cell 1. Thereafter, the liquid crystal is covered with the mother TFT substrate, and the mother TFT substrate and the mother counter substrate are bonded together with the sealing materials 20 and the mother substrate sealing material 2000. In this case, when the amount of liquid crystal to be dropped is too small, air bubbles are generated in the liquid crystal cell 1, and when the amount of liquid crystal to be dropped is too large, liquid crystal enters between the sealing material 20 and the TFT substrate or between the sealing material 20 and the counter substrate, causing sealing defects.
The capacity of the inside of the liquid crystal cell 1 is determined by the height of a columnar spacer 205 formed in the display region. As the height of the columnar spacer 205 is greater, the capacity of the inside becomes large, and as the height of the columnar spacer 205 is smaller, the capacity of the inside becomes small. Accordingly, a proper dropping amount of liquid crystal varies depending on the height of the columnar spacer 205. However, the height of the columnar spacer 205 changes depending on the process.
For addressing the problem, the height of the columnar spacer 205 formed on the counter substrate 200 is conventionally measured in each of the counter substrates 200, the counter substrates 200 are divided into groups according to the height of the columnar spacer 205, and the dropping amount of liquid crystal to be dropped onto the counter substrate 200 is determined in each of the groups.
Although the gap between the TFT substrate 100 and the counter substrate 200 is determined by the columnar spacers 205 in the display region, the gap between the TFT substrate 100 and the counter substrate 200 is conventionally determined by glass fibers 250 in the seal portion. FIG. 15 is a schematic cross-sectional view showing this state. In FIG. 15, a black matrix 202 and an overcoat film 203 are formed on the counter substrate 200 side, and an inorganic passivation film 107 and an organic passivation film 108 are formed on the TFT substrate 100 side. FIG. 15 is a schematic view, and therefore the other layers are not illustrated.
In the display region indicated by DA in FIG. 15, the gap between the counter substrate 200 and the TFT substrate 100 is determined by the columnar spacers 205 and determined in the seal portion by the diameter of the glass fiber 250 mixed in the sealing material. In FIG. 15, liquid crystal 300 is sealed inside the sealing material.
Although a height HS of the columnar spacer 205 varies depending on the process, a diameter GH of the glass fiber 250 is controlled with relatively high accuracy. When the counter substrates 200 are divided into groups according to the height of the columnar spacer 205, they are divided into three groups: a group where the height HS of the columnar spacer 205 is substantially the same as the diameter GH of the glass fiber 250; a group where the height HS of the columnar spacer 205 is smaller than the diameter GH of the glass fiber 250; and a group where the height HS of the columnar spacer 205 is greater than the diameter HG of the glass fiber 250.
FIG. 16 is a cross-sectional view showing a state of the liquid crystal cell 1 of the group where the height HS of the columnar spacer 205 is substantially the same as the diameter GH of the glass fiber 250. In the example of FIG. 16, the amount of the liquid crystal 300 is properly controlled, and therefore the reliability of the seal portion can be maintained at a high level. FIG. 17 is a cross-sectional view of the liquid crystal cell 1 in the group where the height HS of the columnar spacer 205 is smaller than the diameter GH of the glass fiber 250. Since the dropping amount of liquid crystal is determined by the height of the columnar spacer 205, an air bubble 400 is generated in this group like a region A at the periphery shown in FIG. 17.
FIG. 18 is a cross-sectional view of the liquid crystal cell 1 in the group where the height HS of the columnar spacer 205 is greater than the diameter HG of the glass fiber 250. Since the dropping amount of the liquid crystal 300 is determined by the height of the columnar spacer 205, tilting occurs in the seal portion and the substrate warps outward in this group as indicated by a region B at the periphery shown in FIG. 18. Although only the counter substrate 200 is warped in FIG. 18, FIG. 18 is a schematic view, and actually the TFT substrate 100 side also warps.
FIG. 19 is a schematic view showing why tilting of the substrate occurs in the seal portion. Dropping of liquid crystal is conducted under reduced pressure. Liquid crystal is dropped onto each of the liquid crystal cells 1, the TFT substrate 100 and the counter substrate 200 are overlapped each other, the sealing material is cured, and thereafter the TFT substrate 100 and the counter substrate 200 are returned to the air. Since a reduced pressure region 450 is formed between the liquid crystal cell 1 and the liquid crystal cell 1, that is, between the sealing material and the sealing material, the substrate is deformed inward by atmospheric pressure indicated by open arrows. On the other hand, since the liquid crystal 300 is excessively dropped and sealed in a region inside the sealing material, that is, on the liquid crystal cell 1 side, the substrate is deformed outward. Accordingly, tilting of the substrate occurs in the seal portion.
When the mother substrate 1000 in this state is separated along the scribe line 2, the liquid crystal cell 1 has a cross-sectional shape shown in FIG. 18. When the TFT substrate 100 or the counter substrate 200 is reduced in thickness by polishing, the deformation of the substrate shown in FIG. 18 is likely to occur. As described above, when the liquid crystal 300 is excessively sealed, the reliability of the seal portion is impaired, and the contrast is reduced by a change in gap between the TFT substrate 100 and the counter substrate 200 at the periphery of the display region.